Electronic devices of this type have already been described in the state of the art.
Japanese Utility Model No. 60-183896, published on 6 Dec. 1985, discloses an electronic apparatus fitted with means for measuring and displaying a depth of immersion in water, including, in particular, a pressure sensor. Because of the high electric energy consumption linked to operation of the pressure sensor, means for detecting the presence of water were provided on the apparatus to control the power supply to the pressure sensor, in order to interrupt said supply when the person wearing the apparatus is not in water. Thus, the solution proposed consists in using a capacitive sensor including a transparent electrode arranged on the inner face of the glass. This capacitive sensor causes a capacitance to appear when water is in contact with the glass, which causes modification of a reference signal passing through electronic processing circuits. Thus, the pressure sensor is only supplied with energy when water has been detected in contact with the glass of the apparatus.
It should be noted first of all that numerous devices are known wherein ohmic type means for detecting the presence of water are implemented. These detection means, although having generally reasonable electric power consumption, have a significant drawback, in that they require more or less complex construction. In particular, an aperture has to be provided in the case of the device, which can raise problems of water resistance. For this reason in particular, the Applicant has preferred to concern himself with improving capacitive type detection systems, which remove the need to make a specific aperture in the case of the device.
The technical solution presented in the aforecited Japanese Utility Model has, however, a certain number of drawbacks. In particular, a main switch is provided for controlling the power supply to the pressure sensor, operating in conjunction with the means for detecting the presence of water. In a certain way the water presence detection means fulfil the role of a secondary switch. The direct consequence of the presence of the main switch is that it is impossible to carry out pressure measurements when the apparatus is not immersed in water, which can be useful in certain cases as will be explained hereinafter. Moreover, the location kept for arranging the main switch is such that the water presence detection means are permanently powered from a clock signal. These detection means are thus responsible for a long-term waste of energy insofar as a test for the presence of water is permanently occurring and, in that the function of the secondary switch of the means for cutting the power supply to the pressure sensor is downstream of the detection means.
It should also be noted that the solution proposed does not take account of the fact that the structure described for the capacitive sensor has a stray capacitance, which imposes selection of a relatively high value for the capacitance formed at the glass of the apparatus, to allow the detection means to operate efficiently. Consequently, an additional drawback thus arises from the fact that the electric power consumption of the detection means is high because of the high value of the capacitance. The value chosen for the capacitance has to be even higher given that the stray capacitance is capable of varying as a function of the environmental conditions to which the apparatus is subjected, particularly as a function of temperature. Likewise, since detection of the presence of water occurs by detecting modifications of the reference signal, and since the amplitude of these modifications varies with the value of the capacitance that appears because of the presence of water, said capacitance has to have a value sufficient to make the amplitude of the modifications detectable by the electronic processing circuits.
Moreover, the value of the stray capacitance is capable of fluctuating over the long term, which can give rise to a malfunction of the detection means, in particular because the measurements carried out are absolute and not comparative.